This invention relates to hydrocracking, and more particularly to second stage hydrocracking employing multiple reaction zones.
Fuel demands are increasing worldwide. The fuels produced must meet stringent standards concerned with environmental quality. The most abundant feedstocks currently available are relatively heavy, such as vacuum gas oil and Fischer-Tropsch streams. Hydrocracking is used to convert heavy hydrocarbon feedstocks into lighter materials which may be used to make middle distillate products.
Hydrocracking is typically performed in one or more staged hydrocracking units that can be independent reactors or combined into multi-staged reactors. All hydrocracking processes aim to maximize yield and minimize recycle volume. In most cases, however, yield maximization results in increased recycle, and vice versa.
U.S. Pat. No. 5,705,052 discloses a process for hydroprocessing liquid petroleum and chemical streams in a single reaction vessel containing two or more hydroprocessing reaction stages. Both feedstock and treat gas flow co-currently in the reaction vessel. The whole partially converted hydrocarbon effluent passes to the next reaction zone after being stripped of its xe2x80x9cdissolved gaseous materialxe2x80x9d.
U.S. Pat. Nos. 5,720,872 and 6,103,104 are variations of the process described in U.S. Pat. No. 5,705,052. In U.S. Pat. No. 5,720,872, the major difference is the addition of a multi-staged stripper in a single stripper vessel. U.S. Pat. No. 6,103,104 employs the concept of interbed quench between the hydroprocessing stages.
U.S. Pat. No. 6,017,443 discloses a process for catalytic hydroprocessing, in which a feedstock is introduced at the top of the lower reaction zone for downward flow through and reaction with the catalyst therein. In one embodiment, a partially reacted liquid effluent is pumped from the lower reaction zone to the top of the upper reaction zone for downward flow through and reaction with the catalyst in that zone. The recycle is not fractionated into product and unconverted material prior to recycling, however.
U.S. Pat. No. 4,082,647 discloses a hydrocracking process with two reactors operating in parallel rather than in series. Two different feedstocks may be hydrocracked to maximize distillate production. The second feed is mixed with the vaporous phase from separation of effluent from the conversion of the first feedstock.
U.S. Pat. No. 4,197,184 discloses a conventional multiple-stage process for hydrorefining and hydrocracking a heavy hydrocarbonaceous charge stock. In the process, hydrocracked effluent is admixed with hydrorefined effluent and the combination separated into a hydrogen rich vaporous stream and normally liquid material. The cooled vapor stream is then used as a source of hydrogen and as a quench fluid for both the hydrorefining reaction zone and the hydrocracking reaction zone.
U.S. Pat. No. 6,106,695 discloses a process having more than one hydrocracking reaction zone which contains hydrocracking catalyst, wherein the catalyst is rejuvenated or reactivated while the process unit remains on-stream by the periodic exposure of partially spent catalyst to hot recycle gas containing hydrogen. The reactors in this process operate in parallel rather than in series.
The instant invention comprises a hydroprocessing method having at least two stages. The first stage employs a hydroprocessing catalyst which may contain hydrotreating catalyst, hydrocracking catalyst, or a combination of both. The second stage employs a series of fixed bed reaction zones, with feed and hydrogen in co-current flow, with inter-bed removal of gas and products. Gas and product removal occur in a flash separation zone in which hydrogen preferably enters countercurrently.
The process of the instant invention maximizes middle distillate yield while minimizing the volume of recycle. Per-pass conversion is defined as fresh feed converted in a stage divided by total feed to a stage. The per-pass conversion rate in each reactor vessel remains low, 40% or less, while the overall conversion rate is 60% or greater.
The process of this invention provides economy in equipment employed. Single bed reactors, which are smaller, have lower capacity, and are easier to maintain than multiple bed reactors, may be used. The use of small, single bed reactors provides flexibility in second stage operation. They are of simple design and do not require quench gases or liquids. This promotes economic operation.
The hydroprocessing method of the instant invention, which has at least two reaction stages, comprises the following steps:
(a) passing a hydrocarbon feed into a first reaction stage, which is maintained at hydroprocessing conditions, where it is contacted with a catalyst in a fixed bed and at least a portion of the feed is converted;
(b) combining the effluent of step (a) with product material from the second reactor stage and passing the combined stream to a separation zone;
(c) separating the stream of step (b) into an unconverted liquid effluent and at least one converted stream comprising products having a boiling point below that of the feed;
(d) passing the unconverted liquid effluent from step (c) to a second reaction stage, said stage comprising a plurality of reaction zones, wherein each zone is maintained at hydrocracking conditions and separation occurs between each zone;
(e) contacting the feed in the first reaction zone of step (d) with a catalyst in a fixed bed, thereby converting at least a portion of the feed;
(f) separating the effluent of step (e) into an unconverted liquid effluent, and a hydrogen-rich converted stream;
(g) recycling the hydrogen-rich converted stream of step (f) to combine with the effluent of step (a);
(h) passing the unconverted liquid effluent from step (f) to a second reaction zone of the second stage, the zone being maintained at hydrocracking conditions;
(i) contacting the feed in the second reaction zone of step (h) with a catalyst in a fixed bed, thereby converting at least a portion of the feed;
(j) fractionating the effluent of step (i) to produce gas, naphtha, and one or more middle distillate product streams, unconverted material being recycled to step (d).